As computers and other electronic devices become more ubiquitous, touch-sensing systems are becoming more prevalent as a means for inputting data. For example, touch-sensing systems may now be found in workshops, warehouses, manufacturing facilities, restaurants, on hand-held personal digital assistants, automatic teller machines, casino game-machines, and the like.
Capacitive touch sensing is one of the most widely used techniques in touch screen industries. Capacitive touch sensors are mainly divided in two groups, namely, the continuous capacitive sensors, and discontinuous (patterned) capacitive sensors. In a continuous capacitive sensor, the sensor consists of a sheet of conducting thin film that is excited from four corners. The signals induced by a touch are transmitted from the corners to a controller, where they are decoded and translated to coordinates. In a typical patterned capacitive touch screen the sensor consists of a series of parallel conductive bars that are driven from both ends with an excitation signal from the controller. The signals induced by a touch are transmitted to the controller with the same lead lines that excite the sensor bars. These signals are then decoded in the controller and the touch coordinates are reported to a computer. For examples of this type of sensor refer to U.S. Pat. No. 5,650,597, and U.S. patent application Ser. No. 10/176,564.
In patterned capacitive screens different methods are used to calculate the touch coordinates. For example, in capacitive screens using near field imaging (NFI) the coordinate along the parallel bars, the X-axis, is determined by ratio of the voltage drops on left and right of the touch. The coordinate in the direction perpendicular to the bars, the Y-axis, is determined by finding the location of the touched bar combined by interpolation methods.
Touch sensors utilizing more than one patterned sensing layer can be used to determine the coordinates of a touch with high accuracy in both directions, provided that the sensing layers have the proper pattern geometry. Examples of multi-layered capacitive touch sensors are U.S. Pat. No. 4,686,332, and U.S. Pat. No. 6,137,427. The challenge of capacitively coupling to more than one sensing layer has forced the designers either to use very narrow conducting bars, or to use bars with vastly different widths on the two layers. The former design suffers from a very low signal level, and the latter design has significant error due to sparse distribution of the bars.